Among the various metal oxide catalyst materials for use as cathode components in metal-air cells, manganese oxide catalysts have been investigated due to their low cost and high catalytic activity for oxygen reduction. Manganese can exhibit a number of different oxidation states. Due to the stability of these oxidation states, including 2+, 3+ and 4+, a single composition may contain a stable mixture of a variety of different oxides such as, e.g., MnO2, Mn2O3, Mn3O4, and MnO. The use of manganese oxides, in a variety of oxidation states, as an active catalyst material of a cathode has been reported. See U.S. Pat. No. 4,595,643 (Koshiba et al.) and U.S. Pat. No. 4,892,637 (Sauer et al).
Manganese oxides when used in electrochemical cells have presented several challenges as active catalyst materials. Chief among those challenges is limited catalytic activity. Higher voltage in the metal-air cell depends upon rapid oxygen reduction at the cathode. Such oxygen reduction kinetics, being catalyst limited, allow the cells to be used for low or moderate power applications, such as hearing aids for users with moderate hearing loss, but limits their use in more demanding high power applications.
Foremost among the factors causing polarization in cells employing manganese oxides as active catalyst material is the buildup of peroxides at the electrode due to slow reaction kinetics of both reduction of peroxides to hydroxyl ions and decomposition of peroxides to water and oxygen at the cathode. During manganese oxide catalyzed reduction of oxygen, peroxides, which form upon oxygen reduction, may either decompose into water and adsorbed oxygen or undergo additional reduction to hydroxyl ions, the desired redox reaction. Slow or wasteful decomposition inhibits the desired reduction reaction from peroxides to hydroxyl ions, which can lower the voltage produced by conventional metal air cells.
Manganese oxide contained in an octahedral molecular sieve structure is an efficient catalyst for the decomposition of hydrogen peroxide. See Zhou et al., Journal of Catalysis, 176, 321-328 (1998). A catalyst which efficiently removes hydrogen peroxide may be advantageous because the peroxide is an intermediate in the reduction path of oxygen to hydroxyl ions. This advantage allows the cell voltage to better reflect the full potential of the oxygen reduction half cell reaction for a given current density, thus resulting in a high power cathode. Recently, manganese oxides contained in an octahedral molecular sieve structure such as the hollandite structure have been explored because of their high catalytic activity for peroxide reduction. Zhang and Zhang describe an all solid-state galvanic cell having a cathode comprising nanostructured MnO2/mesocarbon microbeads (MCMB), a compacted zinc anode made from Zn powder and a PTFE binder, and a polymer gel electrolyte comprising a potassium salt of poly(acrylic acid). See “MnO2/MCMB electrocatalyst for all solid-state alkaline zinc-air cells,” Electrochemica Acta, 49 (2004) 873-877. The composition of the MnO2 component of the MnO2/MCMB cathode is said to have been KMn8O16 and the authors report that XRD analysis indicated that it had a hollandite structure. In preparation of the cathode, a 4:1 wt./wt. MnOx/MCMB composite was synthesized by preparing a suspension of MCMB (specific surface area=3.5 m2/g) and a solution of MnSO4.H2O in distilled water. K2S2O8 was added and the mixture refluxed until the pH decreased to about 0.5. The cathode of Zhang and Zhang was tested under idealized conditions, such that its practical use in a commercial battery is difficult to determine, i.e., the cathode was tested in a flooded half-cell and in the presence of a large excess of KOH electrolytic solution.